The present invention relates generally to columns in which mass transfer and heat exchange occur and, more particularly, to contact trays for use in such columns to facilitate interaction between fluid streams flowing within the columns.
As used herein, the term “mass transfer column” refers to a column in which mass transfer and/or heat exchange occur. Examples of mass transfer columns include distillation, absorption, stripping, and extraction columns.
Various types of trays are commonly used in mass transfer columns to promote contact and mass transfer between ascending and downwardly flowing fluid streams. The ascending fluid is typically vapor and the descending fluid is typically liquid, although liquid-liquid and gas-liquid systems are also known. Each tray normally extends horizontally across substantially the entire horizontal cross section of the column and is supported around its perimeter by a ring welded to the inner surface of the circular column wall or shell. A number of trays are positioned in this manner with a uniform vertical spacing between adjacent trays. The trays may be located in only a portion of the column to perform one part of a multi-step process occurring with the column. Alternatively, the trays may fill substantially the entire open area within the column.
Trays of the type described above contain one or more downcomers that are positioned at openings in the tray deck to provide passageways for liquid to descend from one tray to an adjacent lower tray. Prior to entering the downcomer, the liquid on the tray deck interacts with ascending vapor that passes through openings provided in selected portions of the tray deck. Those areas of the tray deck containing vapor openings are commonly referred to as “active” areas because of the vapor and liquid mixing and frothing that occurs above those areas of the tray.
The vapor openings in the tray deck can be simple sieve holes or can be formed as part of fixed or moveable valves. Conventional valves have valve covers supported over the opening in the tray deck by attachment legs. In fixed valves, the valve cover is fixed against vertical movement. In moveable valves, the valve cover is able to move up and down in response to variations in vapor or fluid flow rates through the opening. The valve cover functions to shield the opening in both the fixed and moveable valves against undesirable weeping of fluid downwardly through the opening and to laterally direct the vapor or other fluid that has ascended through the opening for enhanced interaction with the fluid flowing along the tray deck. While a variety of fixed and moveable valves are known, further improvements in their design and performance are desired.